Follow-up Observations of the Second and Third Known Pulsating Hot DQ White Dwarfs

We present follow-up time-series photometric observations that confirm and extend the results of the significant discovery made by Barlow et al.(2008) that the Hot DQ white dwarfs SDSS J220029.08-074121.5 and SDSS J234843.30-094245.3 are luminosity variable. These are the second and third known members of a new class of pulsating white dwarfs, after the prototype SDSS J142625.71+575218.3 (Montgomery et al. 2008). We find that the light curve of SDSS J220029.08-074121.5 is dominated by an oscillation at 654.397+-0.056 s, and that the light pulse folded on that period is highly nonlinear due to the presence of the first and second harmonic of the main pulsation. We also present evidence for the possible detection of two additional pulsation modes with low amplitudes and periods of 577.576+-0.226 s and 254.732+-0.048 s in that star. Likewise, we find that the light curve of SDSS J234843.30-094245.3 is dominated by a pulsation with a period of 1044.168+-0.012 s, but with no sign of harmonic components. A new oscillation, with a low amplitude and a period of 416.919+-0.004 s, is also probably detected in that second star. We argue, on the basis of the very different folded pulse shapes, that SDSS J220029.08-074121.5 is likely magnetic, while SDSS J234843.30-094245.3 is probably not.Comment: 12 pages, 19 figures, accepted for publication in Ap

Follow-up Studies of the Pulsating Magnetic White Dwarf SDSS J142625.71+575218.3

We present a follow-up analysis of the unique magnetic luminosity-variable carbon-atmosphere white dwarf SDSS J142625.71+575218.3. This includes the results of some 106.4 h of integrated light photometry which have revealed, among other things, the presence of a new periodicity at 319.720 s which is not harmonically related to the dominant oscillation (417.707 s) previously known in that star. Using our photometry and available spectroscopy, we consider the suggestion made by Montgomery et al. (2008) that the luminosity variations in SDSS J142625.71+575218.3 may not be caused by pulsational instabilities, but rather by photometric activity in a carbon-transferring analog of AM CVn. This includes a detailed search for possible radial velocity variations due to rapid orbital motion on the basis of MMT spectroscopy. At the end of the exercise, we unequivocally rule out the interacting binary hypothesis and conclude instead that, indeed, the luminosity variations are caused by g-mode pulsations as in other pulsating white dwarfs. This is in line with the preferred possibility put forward by Montgomery et al. (2008).Comment: 11 pages in emulateApJ, 12 figures, accepted for publication in Ap

Pulsation in carbon-atmosphere white dwarfs: A new chapter in white dwarf asteroseismology

We present some of the results of a survey aimed at exploring the asteroseismological potential of the newly-discovered carbon-atmosphere white dwarfs. We show that, in certains regions of parameter space, carbon-atmosphere white dwarfs may drive low-order gravity modes. We demonstrate that our theoretical results are consistent with the recent exciting discovery of luminosity variations in SDSS J1426+5752 and some null results obtained by a team of scientists at McDonald Observatory. We also present follow-up photometric observations carried out by ourselves at the Mount Bigelow 1.6-m telescope using the new Mont4K camera. The results of follow-up spectroscopic observations at the MMT are also briefly reported, including the surprising discovery that SDSS J1426+5752 is not only a pulsating star but that it is also a magnetic white dwarf with a surface field near 1.2 MG. The discovery of $g$-mode pulsations in SDSS J1426+5752 is quite significant in itself as it opens a fourth asteroseismological "window", after the GW Vir, V777 Her, and ZZ Ceti families, through which one may study white dwarfs.Comment: 7 pages, 4 figures, to appear in Journal of Physics Conference Proceedings for the 16th European White Dwarf Worksho

On the formation of hot DQ white dwarfs

We present the first full evolutionary calculations aimed at exploring the origin of hot DQ white dwarfs. These calculations consistently cover the whole evolution from the born-again stage to the white dwarf cooling track. Our calculations provide strong support to the diffusive/convective-mixing picture for the formation of hot DQs. We find that the hot DQ stage is a short-lived stage and that the range of effective temperatures where hot DQ stars are found can be accounted for by different masses of residual helium and/or different initial stellar masses. In the frame of this scenario, a correlation between the effective temperature and the surface carbon abundance in DQs should be expected, with the largest carbon abundances expected in the hottest DQs. From our calculations, we suggest that most of the hot DQs could be the cooler descendants of some PG1159 stars characterized by He-rich envelopes markedly smaller than those predicted by the standard theory of stellar evolution. At least for one hot DQ, the high-gravity white dwarf SDSS J142625.70+575218.4, an evolutionary link between this star and the massive PG1159 star H1504+65 is plausible.Comment: 4 pages, 2 figures. To be published in The Astrophysical Journal Letter

SDSS J142625.71+575218.3: A Prototype for A New Class of Variable White Dwarf

We present the results of a search for pulsations in six of the recently discovered carbon-atmosphere white dwarf ("hot DQ") stars. On the basis of our theoretical calculations, the star SDSS J142625.71 + 575218.3 is the only object expected to pulsate. We observe this star to be variable, with significant power at 417.7 s and 208.8 s ( first harmonic), making it a strong candidate as the first member of a new class of pulsating white dwarf stars, the DQVs. Its folded pulse shape, however, is quite different from that of other white dwarf variables and shows similarities with that of the cataclysmic variable AM CVn, raising the possibility that this star may be a carbon-transferring analog of AM CVn stars. In either case, these observations represent the discovery of a new and exciting class of object.NSF AST-0507639, AST-0602288, AST-0607480, AST-0307321Astronom